Author

Date Awarded

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Harry V. Wang

Abstract

Shallow coastal bays and lagoons (mean depths <2-3 meters) are important buffer zones and links between terrestrial and deep marine ecosystems. They are inherently vulnerable to eutrophication, and are normally dominated by benthic primary producers such as seagrass, benthic micro- and macroalgae. There is an urgent need for quantitative models that are specifically designed for studying eutrophication dynamics in shallow coastal ecosystems. In this study, a hydrodynamic and water quality modeling system consisting of the hydrodynamic model UnTRIM and the water quality model CE-QUAL-ICM was applied to a representative shallow coastal bay ecosystem, the Maryland and Virginia Coastal Bays (MVCBs). A high-resolution unstructured model grid was generated to resolve the complex geometry. to address the important role played by benthic macroalgae, a benthic macroalgal module, which assimilated macroalgal kinetics from literature and recent laboratory studies, was incorporated into the water quality model framework. The module includes two representative macroalgal species, Ulva lactuca and Gracilaria vermiculophylla , common in the MVCBs, and employs the internal nutrient-limited growth kinetics proposed by Droop. The numerical modeling system has been calibrated against a comprehensive field monitoring data collected by the Maryland Department of Natural Resources in the MVCBs. The data include water level, current velocity, salinity, and major water quality variables, such as chlorophyll a, dissolved oxygen, and nutrients. The calibrated hydrodynamic model was used to calculate the physical transport time scales. The model estimated flushing time for the entire system is on the order of 2-3 months, which are much longer than typical time scales required by most biological processes. In addition, the local residence time is found to be extremely variable throughout the system. Depending on locations, the local residence time can vary from 0 to more than 200 days. The calculated transport time scales were further compared with spatial water quality distributions in the system. The comparisons demonstrate that physical circulations could substantially modulate biological processes in the system. By using the Droop equation, the benthic macroalgae's unique property, the so-called luxury uptake, was satisfactorily captured. Furthermore, the characteristic boom-and-bust life cycle of benthic macroalgae was qualitatively simulated using a box model. The expanded water quality model that includes the benthic macroalgal module reproduced both temporal and spatial distributions of observed benthic macroalgae and major water quality variables reasonably well in the MVCBs. The model results indicate that benthic macroalgae are highly important in regulating ecosystem metabolism in areas where they are abundant. Moreover, spring phytoplankton bloom was substantially suppressed when benthic macroalgae were present. The incorporation of a benthic macroalgal module also improved the model's predictive capability in simulating dissolved oxygen in shallow ecosystems affected by benthic macroalgae. In terms of nutrient budget, the model estimated that benthic macroalgae retain approximately 10% of annual nonpoint source nitrogen inputs from the watershed based on the simulation of year 2004. This is lower than that contributed by benthic microalgae reported in other shallow coastal bays such as the Lynnhaven Bay. It is suspected that the restricted distribution of benthic macroalgae in the MVCBs limited their role from the whole bay perspective. With the incorporation of a benthic macroalgae module, the overall water quality model prediction capability is improved.